C43A-01 INVITED
Recent Surface Changes in Southern Greenland Outlet Glaciers from NASA's Airborne Topographic Mapper Experiments
In summer 2008, NASA deployed its Airborne Topographic Mapper (ATM), along with other airborne science
instruments, to several locations around the southern periphery of the Greenland ice sheet, with the aim of
quantifying recent changes in a number of outlet glaciers. The ATM also joined with a Swansea University
(United Kingdom) team in order to provide geodetic reference ties on the bare bedrock surrounding a dozen
outlet glaciers, most of them in southeastern Greenland. The goal of this phase of the effort was to make
possible the computation of volumetric change of these glaciers using a decades-long series of oblique aerial
photographs, combined with the geodetic reference provided by the ATM's rock overflights. Finally, we
supported a National Science Foundation and University of Kansas effort to fly a large, dense grid over the
greater Jakobshavn basin to obtain bedrock topography using their 150 MHz depth-sounding radar. In the
process of this, we obtained our own extensive surface-topography measurements over the same grid. Here
we present initial results from these efforts, including changes in the surface topography of the Jakobshavn
and Helheim glacier basins, and along the flowlines of Kangerdlugssuaq and other outlet glaciers in the
southeast of Greenland. Finally we summarize initial, or baseline, measurements along the flowlines of a
number of glaciers never mapped before by the ATM, which we flew as part of our other efforts and which will
pay dividends when the ATM returns to refly the lines in future campaigns.
http://atm.wff.nasa.gov/
C43A-02
A new high-resolution assessment of Greenland ice sheet surface mass balance: 1957- 2008
Large uncertainties remain in the surface mass balance (SMB) of the Greenland ice sheet. To assess the current state and variability of the SMB components we use the Regional Atmospheric Climate Model (RACMO2.1). A model integration for the period 1957-2008 is performed at an unprecedented high horizontal grid resolution of 11 km, which is sufficient to resolve the large melt gradients in the narrow ablation zone and study in detail processes such as percolation, retention and refreezing of melt water. The model results show excellent agreement with observations. Comparison of SMB results with other regional climate models show similar spatial patterns, but locally substantial improvements. Increasing the horizontal resolution enhances the gradients in the SMB and its components. For example, the orographically forced precipitation amount in the south-east of Greenland is much larger than previously concluded. Newly gained surface mass balance observations in this area support this. The integrated ablation and subsequent surface runoff is comparable to other studies, leading to a more positive surface mass balance for the Greenland ice sheet.
C43A-03
Greenland Ice Sheet Annually-resolved Accumulation Rates (1958-2007), a Spatially Calibrated Model
The Greenland Ice Sheet (GIS) has responded dramatically to recent temperature increases, making it an important contributor to sea level rise. Accurate predictions of Greenland's future contribution to sea level will require a scrupulous understanding of the GIS system and refining our understanding of accumulation is a critical step towards this goal. The most accurate existing estimates of Greenland accumulation rates are multi-year averages; existing annual estimates contain poorly quantified uncertainties. This project developed a superior Greenland accumulation dataset that is spatially comprehensive, has annual resolution, is calibrated to field observations and contains sound uncertainty estimates. Accumulation output from a 1958- 2007 run of the Fifth Generation Mesoscale Model modified for polar climates (PMM5) was calibrated against 133 firn cores and coastal meteorological stations. PMM5 accumulation rate estimates contained spatially dependent systematic biases that were modeled and removed using spatial interpolation of zonally derived regressions. The calibrated accumulation dataset contains residual uncertainties exhibiting a strong spatial pattern that was modeled to estimate ice-sheet wide uncertainty. No significant 1958-2007 trends in Greenland accumulation are evident. Average annual accumulation rate is estimated at 0.339m.w.e. or 593km3 with an RMSE uncertainty of +/-83 km3 or +/-14%. The accumulation dataset will be made publicly available.
C43A-04 INVITED
Greenland and Antarctic mass balance from GRACE
GRACE measurements of time variable gravity provide a unique tool to monitor ice mass variability. Using time variable gravity we can directly measure ice mass variations and provide an independent way to monitor ice sheet variability. Using measurements from the GRACE satellites we determine long term and seasonal mass variations of the Antarctic and Greenland ice sheets for over a six year period starting in Apr 2002. During this period both ice sheets displayed significant mass loss. We will analyze the current progress and uncertainties in the GRACE ice mass estimates and we will examine and interpret differences in mass balance estimates from different processing techniques, eg, mascons and harmonics, as well as the effects that various filtering technique have on the final results.
C43A-05
Seasonal and Interannual Evolution of the Earth's Land Ice from GRACE MASCON Solutions
Recent changes in the cryosphere highlight the importance of methods for directly observing the complex spatial and temporal variation of land ice mass flux. Since its launch in March of 2002, the NASA/DLR Gravity Recovery and Climate Experiment (GRACE) mission has been acquiring ultra-precise inter-satellite K-band range and range-rate (KBRR) measurements providing new observations of the complex spatial and temporal evolution of the Earth's land ice. Employing a surface mass concentration (mascon) solution technique, we have computed multi-year time series of surface mass flux for Greenland and Antarctica coastal and interior ice sheet sub-drainage systems as well as the Alaskan glacier systems. These mascon solutions provide important observations of the seasonal and inter-annual evolution of the Earth's land ice. Ice Sheet and glacier system seasonal net balances are quantified. The data show large mass losses from the Gulf of Alaska glacier system during the 2004 summer balance season (-374 +/- 14 Gt) and net balance year (-143 +/- 20 Gt) in response to the 2004 record heat-wave experienced in this region. Large mass losses of the Greenland ice sheet during the 2005 and 2007 summer balance seasons are observed with the largest mass loss (-501 +/- 29 Gt) experienced during the 2007 melt season. Coupled with a low mass gain during the 2006 winter balance season, the Greenland ice sheet experienced a significant mass loss over the 2006-2007 balance year. In addition to these notable regional mass changes, the tip of the western Antarctic peninsula is currently experiencing significant mass loss with a recent net balance of -43 +/- 4 Gt/yr. In this talk we present our latest mascon solutions for the Greenland and Antarctica ice sheets as well as for the Alaska mountain glaciers, including the most recent summer 2008 seasonal balance. We compare our mascon solutions to ICESat and airborne laser altimeter observations of surface elevation change as well as surface melt observations derived from MODIS data. We also quantify the effects of solution technique and background modeling (e.g. atmospheric, tidal, ocean and hydrological mass flux) on the land ice mascon solutions.
C43A-06
Rapid crustal uplift due to unloading of ice from the main outlet glaciers in Greenland
The main outlet glaciers in Greenland have more than doubled their ice volume loss in the past decade. Ice volume loss due to thinning of glaciers would result in a rapid mass unloading of the earth's crust. The elastic adjustments of the lithosphere is detectable using geodetic observations. Here, we use continuous Global Positioning System (GPS) measurements to study vertical crustal motions. We analyze data from ~20 GPS receivers, all located along the edge of the Greenland ice sheet. The rapid unloading of ice from the southeastern sector of the Greenland ice sheet causes an elastic uplift of ~12 mm/yr at a GPS site in Kulusuk (a settlement located ~50 km from the ice sheet margin) and 16 mm/yr at a GPS site in Isortoq (located few km from the ice sheet margin) and 20 mm/yr at HEL2 (a GPS site near the front of the Helheim Glacier). The GPS observations can be explained as due to ice volume loss of ~150 km3 yr-1 due to thinning in the southeastern sector of the Greenland ice sheet (including the Helheim glacier and the Kangerdlugssuaq glacier). Additionally, data from 4 continuous GPS receivers located between 0-150 km from the front of Greenland's Jakobshavn Glacier, suggest an annual net loss of 20-25 km3 of ice from Jakobshavn Glacier.
C43A-07
Greenland Ice Sheet Summer Mass Losses from GRACE, Passive Microwave and Modeling Results.
The Gravity Recovery and Climate Experiment (GRACE) mission has provided an invaluable time series of mass balance of the Greenland ice sheet. The mascon approach to solving for gravity variability allows us to determine mass changes at ten-day intervals and on spatial scales on the order of 100 km. However, because the GRACE signal, represents the integrated contributions from dynamic losses, surface losses, and surface gains, isolating the losses due to melt from the losses due to discharge is not possible without additional information. At the same time passive microwave satellite data provide an extensive record of ice sheet melt extent over the last 30 years, but do not provide any information on the mass or volume of surface melt that is actually discharged into the surrounding seas. We combine results from the GRACE mascon solutions and passive microwave-derived melt extent for each of the major climate zones on the Greenland ice sheet and compare these sets of observations to results from surface mass balance analyses. The combination of these sets of results allows us to determine quantitatively from satellite observations the amount summer melt lost each melt season during the 2003-2008 period of GRACE observations. They should also enable us to estimate the dynamic component of the mass loss during the annual melt periods for 2003-2008. Using the derived relationships between GRACE mass loss and passive microwave melt, we then extend our satellite-based estimate of mass loss from surface melt over the entire 30-year passive microwave melt record.
C43A-08 INVITED
Greenland and Antarctic Ice Sheet Mass Balance from ICESat (2003-2007)
In the last few years, a wide range of estimates of the mass balance of the Greenland and Antarctic ice sheets have been published, with some results differing widely by a factor or two or more for essentially the same time periods. New mass change rates derived from ICESat's laser altimeter measurements of elevation change are generally consistent with those derived from GRACE gravity change measurements. Our firn- compaction model is used to account for elevation changes driven by temporal variations in accumulation rate, as well as temperature, and to determine the appropriate density for converting volume changes to mass changes. Generally, Greenland continues to grow inland and thin at the margins, as it was in the 1990's when the ice sheet was nearly in balance. However, the thinning at the margins of Greenland has increased significantly since the 1990's and the ice sheet is now losing mass by about 165 Gt/yr. Greenland's contribution to sea level rise is now 0.45 mm/yr or about 10% of recent sea level rise. In Antarctica, the Antarctic Peninsula and West Antarctica are losing mass and East Antarctica is gaining mass, by amounts similar to those of the 1990's.